1. Field of the Invention
The present invention relates to a method of expanding color image data, in particular, color image data which is compressed after it is separated into lightness (or luminance) information and chromaticity information.
2. Description of the Prior Art
The present inventors proposed a method of realizing compression such that chromaticity information in a block is represented by two chromaticities in U.S. Pat. No. 4,887,151 and in U.S. patent application Ser. Nos. 185,024 (Apr. 21, 1988), and 261,276 (Oct. 24, 1988) as one method of compressing chromaticity information of color image data. This method is based on an assumption that if a compression unit is limited to a given small block, chromaticity information in the block can be sufficiently represented by two chromaticities in terms of the visual sense characteristics of the human eye.
A typical compression method will be described with reference to FIG. 7. In FIG. 7, color image data is divided in units of 4.times.4 pixels, and is converted into a CIE L*a*b* space as a lightness/chromaticity signal. All the image data to be processed are converted into an L*a*b* uniform color space and are normalized in the level range of from "0" to "255".
Part (A) of FIG. 7 shows original signals obtained by L*a*b*-converting a color image represented by R, G, and B primary color data. A mean value L*mean in a block of L* data is calculated. The L* data block is divided into two regions to have a bold line as a boundary using the mean value as a threshold value. Similarly, a* and b* data are also divided using similar boundaries. When each of the divided regions of the a* and b* data is represented by one chromaticity coordinate value, chromaticity data is compressed. More specifically, in part (A) of FIG. 7, a mean value of chromaticities of each region is employed as a representative value of the corresponding region. That is, if L*mean=151 is the mean lightness of this block, a representative chromaticity of a region having a higher lightness (the upper left-hand region in part (A) of FIG. 7) is (a*U, b*U)=(138, 119), and a representative chromaticity having a lower lightness is (a*L, b*L)=(166, 104).
In this manner, chromaticity signals of 16 pixels are represented by two chromaticity signals, thus achieving compression of an information volume. In addition, a compression ratio can be increased.
Lightness information L* also undergoes compression/expansion. Note that in FIG. 7, for L*, an information preservation type compression method such as a predicted encoding method with which expanded information can be completely decoded is employed.
In a conventional method, chromaticity data is expanded as follows. A pixel block of L* data which is decoded based on a similar principle to that in compression is divided into two regions using a mean lightness L*mean=151 as a threshold value. Two decoded representative chromaticities (a*U, b*U)=(138, 119) and (a*L, b*L)=(166, 104) are assigned to the two regions, thus obtaining expanded blocks.
As described above, in FIG. 7, since no distortion occurs in compression/expansion of lightness, the L* data block before compression is completely reproduced. If an information non-preservation type compression method such as vector quantization is employed for the L* data block, the expanded L* data is approximate to the L* data before compression.
However, in this method, the chromaticity information a* and information b* of the reproduced blocks can only take two states, respectively. This means that an unsatisfactorily reproduced image may be depending on according to the type of image to be processed. For example, when an original image is a color character image, its edge portion is not smooth and a good reproduced image cannot be obtained. This is caused by an immediate change in level at the edge portion since there are only two chromaticity levels. When the decoded lightness signal is distorted, a boundary may be changed to impair a reproduced image.